As being self-luminescent, EL devices have high visibility. In addition, they have high impact resistance as being completely solid devices. Therefore, the use of EL devices in various displays as light emitters is being widely noticed.
EL devices are grouped into inorganic EL devices in which are used inorganic compounds as light-emitting materials, and organic EL devices in which are used light-emitting organic compounds. Of those, organic EL devices have been being much studied and expected as light emitters in the coming generations, since they require a greatly reduced level of voltage, they can be easily small-sized, they consume small electric power, and they can emit light in a mode of plane emission.
Known are various structures of organic EL devices having a basic constitution of anode/organic light-emitting layer/cathode and optionally provided with a hole injection and transportation layer and an electron injection layer, such as anode/hole injection and transportation layer/organic light-emitting layer/cathode, and anode/hole injection and transportation layer/organic light-emitting layer/electron injection and transportation layer/cathode, etc.
In those, the function of the hole injection and transportation layer is to inject holes thereinto from the anode and to transport them to the organic light-emitting layer, while that of the electron injection and transportation layer is to inject electrons thereinto from the cathode and to transport them to the organic light-emitting layer. The function of the organic light-emitting layer is to receive holes and electrons and to emit fluorescence through recombination of those holes and electrons.
Various structures of such organic EL devices have heretofore been studied in order to improve their light-emitting efficiency and to prolong their life. For example, proposed was the disposition of a plurality of organic light-emitting layers in EL devices to thereby improve the light-emitting efficiency of the devices. Japanese Patent Application Laid-Open (JP-A) No. 326146/1993 discloses an organic EL device having two organic light-emitting layers. However, the organic EL device disclosed, in which light being emitted from two layers is taken out, is problematic in that its light-emitting efficiency is low, or is in the order of 1 candela/ampere, and that its life is short, or is in the order of 1000 hours (initial luminance: 100 candela/m.sup.2). JP-A No. 326146/1993 discloses nothing relating to a technique of doping organic light-emitting layers with fluorescent substances.
The technique of doping organic light-emitting layers with fluorescent substances is known. For example, JP-A No. 65958/1995 which provides an organic EL device discloses a technique of doping both the organic hole transportation layer and the organic light-emitting layer with a fluorescent substance to thereby prolong the life of the device. However, the organic EL device disclosed is still problematic in that its emission half-life in continuous driving operation is short. JP-A No. 65958/1995 discloses nothing relating to the improvement in the light-emitting efficiency of the device through doping with fluorescent substances.
JP-A No. 213172/1996 discloses an organic EL device having two organic light-emitting layers of which one is doped with a fluorescent substance. However, the organic EL device disclosed is problematic in that its overall light-emitting efficiency is low as the light-emitting efficiency of the non-doped organic light-emitting layer is low.